1. Field of the Invention
The present invention relates to a current control system for a linear solenoid and, more particularly, to a current control system for a linear solenoid, as used for controlling the oil pressure of an automatic transmission to be mounted on a vehicle.
2. Related Art
Generally speaking, a linear solenoid used to control oil pressure is designed to have a proportional relation, as illustrated in FIG. 13, between a current value I (mA) to be applied to the linear solenoid and an oil pressure (P). In short, the oil pressure (P) of a hydraulic circuit increases with the increase in the current value I (mA). A clutch pressure is produced by the oil pressure (P).
A command current value is set for producing the desired clutch pressure according to the running state of the vehicle. The command current value thus set is corrected to an output current in accordance with the difference from the value of the current that is actually flowing through the linear solenoid. Moreover, a voltage value is set on the basis of an output current and the resistance value of the linear solenoid, as stored in a memory unit, so that the voltage is applied to the linear solenoid by a solenoid drive circuit in response to a signal coming from a PWM output part.
The resistance value of the linear solenoid changes according to its ambient temperature. In short, the resistance value of the linear solenoid increases in proportion to the rise in the temperature of the oil in contact with the linear solenoid.
In the prior art described above, however, the output voltage value to the linear solenoid is set by a feedback control on the basis of the corrected output current and the resistance value stored in the memory unit. At a cold run, for example, the oil temperature is so low that the resistance value of the linear solenoid is actually low. However, the resistance value used when the voltage value is to be set is a fixed value retrieved from the memory unit and based on the oil pressure in a steady run. Therefore, the resistance value used is not equal to the real resistance value of the linear solenoid. As a result, the actual current flowing through the solenoid exceeds the command current. In an overheat state, on the other hand, the oil temperature rises to raise the actual resistance value so that the actual current flowing through the solenoid becomes low.
Because the feedback control is performed by monitoring the real current flow, a current value different from the desired target value, is outputted for a long time so that the clutch pressure control is influenced to cause a shift shock. Moreover, this shift shock always occurs under cold run or overheat conditions.